Add field to link a conversation history

[Orbiter]

This shall enable the 'conversation' view and the field is also required for loklak-native tweets

[Orbiter]

if there is a solution, check fossasia/loklak_webclient#122

[Orbiter]

for this, a parent field was already generated.

[Orbiter]

still needs to be tested with the /api/push.json api to see if pushed parent will reach the index

[mariobehling]

@Orbiter Where could we see, if this is working?

[mariobehling]

Closing this issue due to inactivity. Please reopen, when it becomes relevant again.